This study is designed to test the efficacy of aspirin in preventing the recurrence of adenomatous polyps of the large bowel. Adenomatous polyps of the colon are benign neoplasms that are thought to be the precursors of most colorectal cancers. If we could prevent colon adenomas, we could prevent colon cancer. The principal hypothesis of the proposed study is that aspirin 160 mg/day will reduce the recurrence of adenomatous polyps among those with a recent history of these tumors.